1. Field of the Invention
The present invention relates to an image printing method and system.
More specifically, the present invention relates to a printing method and system for improving image quality in dot matrix printing systems, such as inkjet printers.
More specifically, the present invention relates to a printing method using an arrangement of curing stations and print heads in the context of inkjet printing with UV-curable inks.
2. Description of the Related Art
Printing a digital document is one of the most efficient ways to convey information to a user. New print-on-demand technologies such as laser printing and inkjet printing enable to print documents almost instantaneously without the need for creating intermediate printing masters.
Inkjet printing works by jetting ink droplets through a nozzle onto a substrate.
In the case of a continuous inkjet, a continuous stream of electrically charged ink droplets is produced and electromagnetic fields are used to guide this stream away from or towards a substrate so as to form an image on the substrate.
In the case of a drop-on-demand inkjet, a mechanical or thermal energy pulse is applied to ink residing in a small chamber in order to create a pressure wave that propels a miniscule ink droplet at high speed through the nozzle towards a substrate. The pressure wave is controlled by shaping the length and the profile of the electrical waveform that is applied to the thermal or mechanical transducer in the ink chamber. In many cases, the volume of the droplet and the size of the ink spot are substantially fixed. In other cases, the volume of the droplet can be modulated to create ink spots having different sizes on the substrate.
Printing the image of a document is achieved by moving the nozzle relative to the substrate along a raster by a shuttle in combination with a substrate transport mechanism and selectively jetting ink droplets on the substrate in response to the image of the document.
When the ink droplets land on a substrate, they form ink spots. Because these ink spots are small, they cannot be individually resolved by the human eye but together they render a visual impression of the image of the printed document. Generally, a halftoning technique is used to determine the spatial distribution of ink spots that produces an optimal rendering of the image of a given document.
To increase printing speed, usually not one, but an array of nbrNozzles inkjet nozzles are generally used that can be operated in parallel. Such an array of nozzles makes up a print head.
By moving the shuttle with the print head across the substrate in a fast scan orientation, a set of parallel raster lines of pixels can be printed in one step. Such a set of raster lines is called a swath.
When a swath has been printed, the print head is moved in a slow scan direction over a distance of the length of the array of nozzles to print an additional swath of lines underneath the previous swath. This process of printing swaths is repeated until a complete document is printed on the substrate.
The smallest value for the nozzle pitch is practically limited by constraints imposed by the manufacturing process. For reasons of image quality, however, a printing pitch in the slow scan direction is often desired that is smaller than the nozzle pitch. U.S. Pat. No. 4,198,642 teaches that a value can be selected for the printing pitch in the slow scan orientation that is an integer fraction 1/n of the nozzle pitch by using an interlacing technique.
Because of manufacturing tolerances, systematic variations in the volume of droplets and of both their ejection velocity and direction exist between nozzles belonging to the same inkjet head. If all the ink droplets of a single line of pixels in the fast-scan orientation are printed by the same nozzle, the variations in the ejection direction across the slow-scan orientation show up as correlated image artifacts that look like banding or streaking.
U.S. Pat. No. 4,967,203 introduces a technique to resolve this problem. By having the pixels on one and the same line printed by different nozzles instead of by the same nozzle, the correlated image quality artifacts can be de-correlated. The underlying assumption is that the image quality artifacts caused by variations between different nozzles are uncorrelated. De-correlating the image quality artifacts diffuses them over the printed substrate so that they become less perceptible or preferably imperceptible. In many documents, this technique is referred to as shingling. The method presented in U.S. Pat. No. 4,967,203 uses a staggered application of ink dots such that overlapping ink dots are printed in successive passes of the print head.
In U.S. Pat. No. 6,679,583, an improved technique is presented that combines the effects of the teachings in U.S. Pat. No. 4,198,642 and U.S. Pat. No. 4,967,203 and adds a number of other improvements, including improved printing speed. In this document, the term “mutually interstitial printing” is introduced to describe both interlacing and shingling. The term mutually interstitial printing also avoids confusion, as the term shingling is preferably used in the graphic arts industry to describe a technique that compensates for the effects of the thickness of the paper on the width of the margin in saddle-stitched bookmaking.
Once an ink droplet ejected by a nozzle lands on a substrate, it is cured so that it receives the required resistance against rubbing. Ink curing can be achieved by a number of mechanisms.
A first mechanism of ink curing is absorption of the ink into fibers of the substrate or a porous coating. This is the dominant mechanism when oil or water based inks are used.
A second mechanism of ink curing is coagulation of the ink by evaporation of an ink solvent. When the ink solvent has evaporated, pigments or dyes together with a binder material are left on the paper.
In many practical applications, a combination of the two above effects takes place: ink is initially absorbed by a substrate and then, depending on the vapor pressure of the solvent, evaporates in a shorter or longer time.
A third mechanism of ink curing is polymerization, for example, under the influence of an external energy source such as a UV light source. The high-energy radiation creates free radicals that initiate a polymerization reaction that solidifies the ink. The main advantage of this technique is that it enables the printing on media that do not absorb ink.
A fourth mechanism of ink curing is phase or viscosity change by temperature. Ink is jetted at a high temperature when it is in a liquid phase and solidifies when it cools down on the printed surface.
An objective technical problem exists in inkjet printing when the ink spots from different droplets on the substrate touch each other before they are cured. Because of complex physical effects related to surface tension, the touching ink spots may coalesce. This coalescence results in a mottled appearance of tints that are printed. The effect is most pronounced in tints with a high density, because in these tints, the average distance between the spots is shorter and the risk that neighboring ink spots touch is higher.
The problem of coalescence becomes worse in the case of so-called wet-on-wet printing. Wet-on-wet printing is a technique wherein the droplets from different nozzles land on the same position of the substrate without intermediate curing. A typical example is in color printing where up to four droplets with cyan, magenta, yellow, and black ink printed by different heads mounted on the same shuttle can land on the same pixel position. An advantage of wet-on-wet printing is that the final color of a pixel is not heavily affected by the order of printing the droplets because the inks physically mix before they are cured. This property is particularly advantageous in the case of bidirectional printing, because in bidirectional printing, the order of printing droplets by different heads reverses when the slow scan direction reverses. However, the piling up of droplets on the same position on the substrate also greatly increases the risk for coalescence.
A first solution to the problem of coalescence would be to reduce printing speed. By reducing printing speed, more time is available to cure an ink spot before a neighboring ink spot is printed and this reduces the risk of coalescence.
Reducing the printing speed, however, also increases the waiting time for a printed result and negatively affects the productivity of the inkjet printer, i.e., the economic value that the investment in the printer can create over its lifespan.
Another solution would be to increase the distance between the ink spots by making them smaller or by decreasing the resolution of the addressable grid of printable dot positions. This solution, however, negatively impacts the density that can be achieved when a dot is printed at 100% of the printable dot positions. A comparison between FIGS. 16A and 16B shows that when the ratio of the diameter spotDiameter 720 of an ink spot divided by the shortest distance pixelSize 710 between two printable positions becomes smaller than the square root of 2, areas between the spots are left on the substrate that receive no ink. These areas negatively impact the density of the darkest tint that can be achieved with this system.
Yet another solution would be to change the order of the droplet printing. By printing neighboring pixels at different times, the pixels that are printed first can already be cured before the remaining pixels are filled in. This effect is implicitly achieved when the technique is used as described in U.S. Pat. No. 4,967,203. Because different sets of pixels on the same line are printed during different swaths, there is time to cure a set of pixels printed during an earlier swath before a set of pixels of a later swath are deposited. By spreading the deposition of neighboring ink droplets in time, coalescence is reduced and at the same time, correlated image artifacts are diffused. The method is effective at moderate printing speeds. When higher printing speeds are required, however, the method fails to avoid the occurrence of coalescence.
Yet another solution would be to force the curing of ink droplets when they land on the substrate before additional droplets are printed at nearby pixel positions. This would, for example, be achieved by using a UV curable ink and a UV source that is mounted on the same shuttle and that follows the print head. The patent document U.S. Pat. No. 6,092,890 discloses an apparatus that uses a set of print heads for ejecting UV curable ink droplets in combination with a single UV source associated with the set of print heads for curing the inks by hardening or solidifying the ink drops on the receiver. This improves the problem of coalescence but introduces another problem. Hardening the ink drops on the receiver immediately after they are printed results in a surface that becomes microscopically “bumped” in an image-wise fashion. Another effect is that when an ink droplet during a subsequent pass lands at or near a cured ink spot, it tends to spread in a completely different way than when the same droplet would land on a wet droplet or on an unprinted substrate. The result is an image with an uneven gloss and texture. What is really needed is a system that results in even gloss and smooth texture of a printed document. Another problem with the disclosure in the U.S. Pat. No. 6,092,890 is that it provides no clear explanation of the printing method itself. It is not clear, for example, whether in one pass of the print heads one or more inks are deposited at the same time or not. Furthermore, since only a single UV source is used, the apparatus is designed to print only in one direction along the fast scan orientation, which lowers the maximum achievable printing performance compared to systems that support bidirectional printing.
WO 2004/002746 describes a method and an apparatus and introduces the concept of a first “partial curing” step by a first UV source followed by a “final curing” step by a second UV source. The image is reconstructed by printing series of mutually interstitial images with intermediate curing. The partial curing of each mutually interstitial image immediately after printing enables to control the coalescence of ink without substantially compromising the smoothness of the gloss and texture of the final printed surface. Because the method and the apparatus in the document WO 2004/002746 use only one UV lamp for the intermediate curing, they are designed for printing only in one direction along the fast scan orientation, which limits the maximum achievable printing performance compared to systems that support bidirectional printing.
Bidirectional printing has been described in the prior art, however, not in the context of printing techniques that use intermediate curing. Many technical problems that involve the management of printing and curing, the lay out of an apparatus for such purpose, and the required image processing to suppress correlated image artifacts and to achieve a smooth and even gloss and texture of the printed result hence remain unresolved.
In view of the state of the art, an improved and alternative method and apparatus are needed for dot matrix printing that suppresses coalescence, support the printing with UV curable inks, optimizes printing performance, supports bidirectional printing, suppresses correlated image artifacts, and results in an even gloss and smooth texture of the printed result.